Arrowhawk...you really don't understand the premise of the Tier system at all. It's not about pure power. I can build a charger fighter who can deal enough damage to under the earth. That doesn't make a fighter Tier 1, though. Just like your semi truck, its only set up to do one thing....deliver payload.
No, the tier system is a measure of potential power, access to that power, and versatility while wielding that power. The above fighter is a hammer, and its really good at pounding in nails (damage). The high tier classes are more like that Chinese shovel multitool. If yyou can't figure out how to solve your problem with one of those, you aren't thinking hard enough. It can hammer in nails too, about as good as the hammer can, too. It can also dig pits, cut wire, paddle a raft, be used as a grappling hook, saw a log, etc, etc, etc.
A high tier class can deal damage like a low tier class while still maintaining flexibility. It can also do more with less. Less gear, less focus, less optimization. I can end an encounter in one round with a charger, but it takes a careful synergy of 2 classes, 4 feats, and 3 magic items. I can do the same on a wizard by memorizing Glitterdust. Less effort, same results. Or, more effort, better results.
I can build a monk who is highly versatile and a powerful combatant. It requires a lot of work, obscure books, and carefully selected features and gear, and in the end, contains very few levels of the actual monk class. If I don't, I either get something that is strong but shallow, or I get something that is broad, but weak. I can do the same on a Druid, simply by taking Natural Spell at 6. Potential, and level of effort, that is how tiers are measured.
And yea, you were right about one thing. Spells have the highest versatility and the highest return on investment. That's why all Tier 1 and 2s are caster or pseudo-caster. The most versatile caster (Clerics, Druids, Wizards) are the highest, with the less versatile caster (Sorcerers, Psions, Beguilers, Bards, etc) being lower tiers.